1. Field of the Invention
The present invention relates to an implantable intravascular pressure determining device and method.
2. Description of the Prior Art
A cardiac stimulating apparatus is described in U.S. Pat. No. 6,026,324 that non-intrusively determines a value indicative of hemodynamic pulse pressure from an accelerometer signal obtained by an accelerometer sensor enclosed in an implantable casing of the stimulating apparatus. The accelerometer sensor is electrically coupled to a microprocessor-based controller and the accelerometer transmits a signal to the controller associated with fluid and myocardial accelerations of the patient's heart. A filtering arrangement is coupled to the accelerometer for filtering and conditioning the signal transmitted by the accelerometer to produce a waveform related to a pulse pressure within the patient's heart. In order to remove ancillary information contained in the acceleration signal the signal is transmitted through a series of filters. Thus, the above-referenced United States patent discloses a device capable of non-intrusively (meaning that no sensor needs to be inserted into the heart) determines a waveform related to the pressure and in particular the pulse pressure within a patient's heart.
Measuring pressure inside a heart by inserting a pressure sensor into the heart is well-known in the art. One example is given in the background section of U.S. Pat. No. 6,026,324 where it is referred to U.S. Pat. No. 4,566,456 discloses a device that adjusts the stimulation rate relative to right ventricular systolic pressure. The ventricular systolic pressure is measured by a piezoelectric pressure sensor mounted on lead inserted into the heart, i.e. an intrusive pressure measurement technique.
In order to obtain accurate and reliable measurements of the intracardial pressure it is often preferred to perform pressure measurements by arranging a pressure sensor inside the heart.
Intracardiac pressure is a highly valuable parameter for estimation of cardiac condition and cardiac pumping efficiency. Technically there is no difficulty in placing a pressure sensor in e.g. the right ventricle of a heart.
Although the pressure sensor may give a correct picture of the pressure at the sensor site, however, the pressure measured in an active patient is a summation of pressures having different origins. Apart from the desired component i.e. the pressure originating from the heart's pumping action, the sensor signal will contain pressure components from other sources such as vibration, external and internal sounds and barometric pressure changes.
In this context, it is relevant to note, that an 11 meter elevation in air gives rise to a pressure change of 1 mm of Hg. Also, it should be noted that the blood column in the body (in the actual case mainly the blood column in the heart) generates pressure changes when the body is exposed to exercise and/or vibrations.
This may be summarized by the following relationship:p=d.h.a  (Equation 1A)where p is the pressure change, d is the blood density, h is the blood column height and a is the acceleration. It should be noted that in the relationship it is indicated that h and a are vectors.
The same blood column will likewise give rise to pressure changes during body posture changes according to:p=d.h.g  (Equation 1B)where g is the gravity constant.
External and internal sounds also can make a non-negligible contribution to the pressure signal. Examples of such external sounds are traffic noise and loud music and internal sounds such as coughing, sneezing and snoring.
Taking the above into account, it is fairly difficult to extract the desired signal i.e. the pressure signal emanating solely from the heart's pumping action, from the sensor signal.
For many applications it would be sufficient to measure the cardiac pressure during limited time intervals. One issue is then how to find intervals during which the cardiac pressure signal is the dominating signal contributor.